Postmenopausal women are at higher risk of coronary heart disease (CHD) than premenopausal women, even after controlling for age-effects. However, estrogen replacement therapy significantly lowers the risk of CHD in postmenopausal women. With the study, we propose to determine the kinetics of apolipoprotein A-I, the major protein component of high density lipoproteins, in postmenopausal women undergoing a placebo- controlled, randomized treatment with each of the following regimens: a. placebo; b. 0.625mg/d conjugated estrogens; c. 0.625mg/d conjugated estrogens + 2.5 mg/d medroxyprogesterone acetate. The kinetics of apo A-I will be determined by stable isotope techniques. The low dose estrogen (b) and the estrogen-progestin combination (c) are frequently prescribed ro postmenopausal women as hormone replacement therapy, but there are no studies comparing the effects of these two treatments on plasma lipid metabolism and apolipoprotein kinetics.